May 26, 1899.] 



SCIENCE. 



!1 



ments at our disposal so as to complete the 

 survey within a short iuterval, say 10 years 

 at the most, or we must content ourselves 

 for the present with taking a less detailed 

 survey. 



Let us say that our present means will 

 enable us to complete 450 stations per an- 

 num, of which 400 are to lie within the 

 United States. Suppose that at the end of 

 the year 1910 we shall have occupied 4,000 

 stations in the United States and have made 

 the necessary 'repeat observations,' and 

 that the stations have been to some degree 

 uniformly distributed, then we shall have 

 on the average one new station to every 756 

 square miles. Selecting as the epoch to 

 which the observations shall be reduced 

 January 1, 1905, we should then have with 

 the addition of about 1,000 former stations, 

 which we could utilize, a magnetic survey, 

 the stations of which would be distributed 

 at the average rate of one to every 600 

 square miles, or, approximately, one station 

 to an area 25 miles, 40 kilometers, square. 



This will give a very satisfactory repre- 

 sentation of the distribution of the earth's 

 magnetism within our confines and will suf- 

 fice for the accomplishment of many of the 

 practical purposes of magnetic surveys. 



"We will call this our ' first survey ' and, 

 as stated, its epoch 1905. We shall now be 

 able to tell in what portion of the country 

 more stations are needed. That is the 

 density of the ultimate distribution of sta- 

 tions will not be a uniform one. In regions 

 where the distribution of magnetism is 

 fairly regular comparatively lew stations 

 will suffice, while in magnetically disturbed 

 areas the number of stations must be in- 

 creased in uniformity with the character 

 and extent of the disturbance. The subse- 

 quent work will consist then in filling in 

 stations where most needed and repeating 

 observations at the ' repeat stations.' 



In short, the plan of conducting a mag- 

 netic survey of this country which appears 



to be best suited to the present conditions, 

 and one that is possible to carry out within 

 a reasonably short time, is as follows : 

 To make, first, a general magnetic survey of 

 the country with stations about 25 to 30 

 miles apart ; then, as opportunities present 

 themselves, to observe more closely the 

 magnetically disturbed areas. The observa- 

 tions at the ' repeat stations ' made from 

 time to time will furnish the proper secular 

 variation corrections. 



The great advantages of this plan over 

 that of attempting a very detailed magnetic 

 survey at once, the steady progress of 

 which over the entire country, on account 

 of its extent, would necessarily be very 

 slow, will be readily perceived. The plan 

 thus briefly outlined will make it possible 

 within a reasonable time to construct two 

 sets of magnetic maps for the same epoch, 

 each set based upon a different distribution 

 of the stations. An opportunity will thus 

 be afibrded, as in the case of the recent 

 magnetic survey of Great Britain, to obtain 

 some idea of the accuracy with which the 

 iso-magnetic lines can be determined. The 

 satisfactory solution of this question will 

 serve as a valuable guide in future magnetic 

 work. 



Several State Geologists are making plans 

 for detailed magnetic surveys of their re- 

 spective States, in cooperation with the 

 Coast and Geodetic Survey. 



In addition to the observation of the 

 magnetic elements at numerous points it is 

 necessary to maintain a few magnetic ob- 

 servatories where continuous observations 

 over a term of years will aflbrd the data 

 for comparing and reducing observations 

 and for detecting the general changes in 

 the earth's magnetic force. The Coast and 

 Geodetic Survey has a number of years 

 maintained such an observatorj', for a time 

 at Los Angeles and later at San Antonio, 

 at which point the observations were 

 brought to a close, as they have been in the 



